1. Field of the Invention
The present invention relates to particular hematite particles suitable for a non-magnetic undercoat layer of a magnetic recording medium, more particularly, to particular hematite particles containing the total sodium amount of not more than 50 ppm, method thereof and a magnetic recording medium using said particular hematite particles.
2. Description of the Prior Art
With a recent development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and a magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, a higher output characteristic and a lower noise level.
Especially, much higher recording density of audio or video tapes is always desired and carrier signals tend to move to a shorter wavelength range.
Meanwhile, since an influence of self-demagnetization becomes prominent as the recording wavelength becomes shorter, it is necessary to reduce an influence of self-demagnetization by thinning of a magnetic recording layer with a view to higher recording density. That is supported, for example, on page 312 of "Development of Magnetic Materials and Technology for High Dispersion of Magnetic Powder" published by Sogo Gijutsu Center (1982), ". . . the conditions for high-density recording in a coated layer type are that high output characteristic and low level of noise with respect to short wavelength signals are maintained. To meet such conditions, it is required to have large coercive force Hc and residual magnetization Br . . . and to have a thinner thickness of the coated film . . . ".
In light of the foregoing situation, it is proposed and practiced to reduce self-demagnetization by decreasing the thickness of an upper magnetic recording layer and to solve the problems such as a deterioration in surface smoothness and a deterioration in electromagnetic performance by forming on a non-magnetic base film at least one non-magnetic undercoat layer which comprises dispersing non-magnetic particles such as hematite particles in a binder (Japanese Patent Examined Publication (Kokoku) No. 6-93297, Japanese Patent Non-examined Publication (Cokai) Nos. 62-159338, 63-187418, 4-167225, 4-325915, 5-738823, 5-182177, 5-34,7017, 6-60362, etc.)
Moreover, with a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, surroundings in which a magnetic recording medium is used and stored become diversified and it is required to have storage stability not only in ordinary conditions, but in high temperature and high humidity conditions.
As a cause of lowering the electromagnetic performance, the storage stability of a magnetic recording medium and the dispersion stability of a coating composition, a water-soluble alkali metal, in particular, a water-soluble sodium contained in the magnetic recording medium is pointed out.
The Japanese Patent Non-examined Publication (Kokai) No. 9-22524, for example, on page 3, column 3, lines 14-20 discloses; ". . . When a free fatty acid increases and water-soluble Na and Ca contained in non-magnetic particles abound, Na and Ca salts of the fatty acid tend to deposit to thus afford an adverse effect to the electromagnetic performance such as output performance and C/N, but by reducing those salts to the specific amount or less, an excellent storage stability and a low friction coefficient are obtained without a deterioration in electromagnetic performance. The Japanese Patent Non-examined Publication (Kokai) No. 62-209806 discloses on page 2, left upper column, lines 3-19; ". . . the residual Na.sup.+ has been known to have a great influence on the quality of a magnetic coated film. As a typical example, a so-called "salt-depositing phenomenon" is pointed out. That is, when a polyvinyl chloride-based resin is used as part of a binder, crystals of NaCl deposit on the surface of a magnetic recording layer which invites D.O. (dropout) to thus damage the quality of magnetic tapes. Moreover, there are data showing that the above-mentioned phenomenon becomes a cause of "blocking" which is a characteristic of video tapes (Blocking is a phenomenon that when running of a video tape was stopped by power failure or the like with the video tape being loaded on a video deck under high temperature and high humidity conditions, the magnetic recording layer is held to be attached to an upper cylinder of the video deck, in consequence, part of the magnetic recording layer peels off.). Thus, the reduction of the residual Na.sup.+ contained in the magnetic powders has been long-waited." Furthermore, Japanese Patent No. 2641662 discloses on page 2, column 3, line 38 to column 4, line 14; ". . . fatty acids react with alkali metals such as sodium and potassium which are impurities of carbon black to thereby form alkali metal salts of the fatty acids . . . These alkali metal salts of the fatty acids arc insoluble in an organic solvent . . . powder of these alkali metal salts of the fatty acids deposit on the surface of the magnetic recording layer to thus become a cause of dropout . . . the decomposition product of the organic solvent lower adsorptivity of a binder resin to the surface of an inorganic fine particle filler to result in a decrease in mechanical strength of a coated film. In addition, storage stability as a coating composition deteriorates."
It has been reported that an improvement in storage stability of a magnetic recording medium is tried by reducing a water-soluble sodium salt contained in a magnetic recording medium or in non-magnetic or magnetic particles added to the magnetic recording medium (Japanese Patent Non-examined Publication (Kokai) Nos. 62-209726, 62-209806, 9-22524, 9-147350, 9-231546, 9-170003, 10-177714, 10-198948, Japanese Patent Examined Publication (Kokoku) No. 7-82638, Japanese Patent No. 2641662, etc.).
As is discussed above, non-magnetic particles for a non-magnetic undercoat layer have been strongly demanded which are capable of providing a thin magnetic recording layer having a smooth surface and uniform thickness when the magnetic recording layer is formed on the non-magnetic undercoat layer obtained by dispersing the non-magnetic particles in a vehicle, and further, capable of providing a magnetic recording medium excellent in electromagnetic performance and storage stability, but such non-magnetic particles have not been hitherto obtained.
That is, in the above-mentioned Japanese Patent Non-examined Publication (Kokai) No. 9-147350, it is described that the amount of alkali metals contained in non-magnetic particles of a non-magnetic layer is less than 1500 ppm. However, as will be described later as comparative examples, when the non-magnetic particles contain approximately 1500 ppm of alkali metals, dispersibility in a vehicle is poor because of high desorption ratio of resin, and hence, a magnetic recording medium obtained by employing the non-magnetic particles as ones tor a non-magnetic undercoat layer of a magnetic recording medium is weak in strength of a coated film and storage stability is not said to be satisfactory. Moreover, as methods for production of non-magnetic particles having alkali metals of less than 1500 ppm, a method for employing as an alkali aqueous solution. for example, an ammonium aqueous solution not containing alkali metals, and a method for carrying out sufficient washing after the completion of production or before the final heat-treatment are described. However, according to these methods, the total amount of sodium can only be reduced to approximately 100 ppm as described in the publication and can not be reduced to 50 ppm or less. Thus, when such non-magnetic particles are used as ones for a non-magnetic undercoat layer of a magnetic recording medium, it cannot be said that the storage stability of the obtained magnetic recording medium is satisfactory.
In the above-mentioned Japanese Patent Non-examined Publication Nos. 9-22524 and 9-170003, it is described that a soluble sodium contained in nonmagnetic particles of a non-magnetic layer is 0-150 pm or 300 ppm or less. However, as will be described later as comparative examples, according to these methods, a soluble sodium can be reduced to approximately 45 ppm but a difficulty-soluble sodium is contained in an amount of approximately 300 ppm. Since the difficulty-soluble sodium converts into a soluble sodium through moisture contained in air or a coated film and to come out to deposit, when the non-magnetic particles are used as ones for non-magnetic undercoat layer of a magnetic recording medium, the storage stability of the magnetic recording medium can not be said to be satisfactory.